A light emitting device having a light emitting layer section made of (AlxGa1-x)yIn1-yP alloy, wherein 0≦x≦1, 0≦y≦1 (hereinafter also referred to as AlGaInP alloy or simply AlGaInP) adopts a double hetero-structure in which a thin AlGaInP active layer is sandwiched between an n type AlGaInP cladding layer and a p type AlGaInP cladding layer, each with a larger bandgap than the active layer, thereby enabling a high brightness device to be realized. In recent years, a blue light emitting device having a similar double hetero-structure made of InxGayAl1-x-yN, wherein 0≦x≦1, 0≦y≦1 and x+y≦1, has been put into practical use.
FIG. 7A is an example of an AlGaInP light emitting device and in the device 300, a hetero-epitaxial growth is performed on an n type GaAs substrate 1: an n type GaAs buffer layer 2, an n type AlGaInP cladding layer 4, an AlGaInP active layer 5 and a p type AlGaInP cladding layer 6 are stacked in the order to form a light emitting layer section 24 of a double hetero-structure. Numeral symbols 14 and 15 are metal electrodes for applying a drive voltage thereto. Herein, since the metal electrode 14 works as a light interceptor, it is formed, for example, in a way to cover only a central portion of a major surface of the light emitting layer section to thereby extract light from an electrode non-formation area around the electrode 14.
In this case, since an area of a light extraction region formed around the electrode 14 can be increased with reduction in area of the metal electrode 14, a smaller area of the metal electrode 14 is advantageous from the viewpoint of improvement on light extraction efficiency. While an attempt was conducted in the prior art in which a current is effectively spread within a device by an contrivance of a shape of the electrode to thereby increase a light extraction quantity, increase in area of the electrode, in this case as well, was unavoidable one way or another, having leading to a dilenma, to the contrary, in which a light extraction quantity is limited low due to reduction in area of light extraction. Furthermore, a dopant concentration in and, in turn, a conductivity of the cladding layer 6 is restricted to a somewhat low value in order to optimize radiative recombination of carriers in the active layer 5 to thereby produce a tendency of a current being hard to spread laterally. This leads to a phenomenon that a current is concentrated in the electrode covering area to reduce an effective light extraction quantity in the light extraction area. Therefore, a method has been adopted in which a current spreading layer 107 having low resistivity with an increased dopant concentration is formed between the cladding layer 6 and the electrode 14. In a prior practice, as a material of such a current spreading layer 107, there was used, for example an AlGaAs alloy.
While, since the current spreading layer 107 made of an AlGaAs alloy is lattice-matched with an AlGaInP alloy, both layers advantageously can be consecutively grown as a high quality semiconductor layer in a growth furnace, its thickness b, as shown in FIG. 7B, has to be set to a considerably thick value of the order of 50 μm. With such a method adopted, since not only is a time required for film formation longer, but much of raw material also becomes necessary, a productivity is conspicuously reduced to suffer a high cost, having resulted in a great problem in industrial applicability. What's worse, a distance between a surface of the device and the active layer 5, from which light is actually emitted, becomes excessively large to increase series resistance, thereby having produced inconveniences of not only reducing a luminous efficiency, but also degrading a performance in high frequency operation. On the other hand, as shown in FIG. 7C, with decrease in thickness b of the current spreading layer 107, a dilemma arises that the layer becomes short of a current spreading effect to the contrary to reduce an effective light extraction quantity in the light extraction area.
Therefore, a proposal has been made that the entire surface of the current spreading layer 107 made of an AlGaAs alloy is covered with a transparent conductive layer made of ITO (Indium Tin Oxide) with a high conductivity to thereby not only reduce a thickness b of the current spreading layer 107, but achieve a sufficient current spreading effect, with the result of a higher light extraction efficiency acquired.
According to a study conducted by the inventors of the present invention, however, it has been found that in a case where a transparent conductive layer made of ITO is formed on the current spreading layer 107 made of an AlGaAs alloy, a contact resistance between the transparent conductive layer and the current spreading layer 107 becomes high with ease, leading to a state that reduction in a luminous efficiency due to increase in series resistance is hard to be avoided.
It is an object of the present invention to provide a light emitting device capable of improving a light extraction efficiency by adopting not only an oxide transparent electrode layer as an electrode for emission driving, but also a device structure enabling contact resistance of the electrode to decrease.